The present invention is directed to an in situ method of determining the thrust on a valve stem during operation of the valve, without requiring removal of the valve stem to implement the method. This method has particular applicability to determining the thrust applied to a valve stem during seating of a valve disc or body in a valve seat and also during movement of the valve body toward and away from the valve seat.
In any valve, in order to stop the flow of fluid through such valve, the valve disc or body must be properly seated against the valve seat with the appropriate valve stem thrust. This proper seating is all the more critical in high pressure applications.
There are numerous situations in which it is essential that a valve be fully and properly, remotely and quickly closed upon command.
In these applications motorized valve actuators are often employed which, in response to commands from a control center, supply a motive force to the valve stem to close and seat the valve. A motive force which increases in magnitude is supplied to the valve stem until the valve stem supplies a predetermined amount of thrust to the valve disc and the valve seat.
The amount of valve stem thrust required to properly seat a valve is a function of the operating conditions of the valve. For example, in a high pressure valve, a large amount of valve stem thrust is necessary to counteract the high pressure of the fluid flowing through the valve. On the other hand, in a low-pressure application, there would be substantially less pressure present to counteract the thrust applied by the valve stem. In any case, as a function of the operating conditions, the manufacturer of a valve often specifies a minimum amount of valve stem thrust which is required to be applied by the valve stem in order to properly seat a valve in the valve seat.
In common motorized actuators, a torque limiter is often employed to shut down the motive source whenever a predetermined operating condition is reached in the actuator. Ideally this predetermined condition has a direct correlation with the amount of thrust at the valve stem. Unfortunately, the monitored condition is often only an indirect indication of the actual valve stem thrust present at the valve stem. The result is that once a valve, actuator and torque limiter are assembled and operational in a system, there is no direct method of verifying that the exact amount of thrust required to close the valve is in fact being supplied when the actuator shuts off.
It has been found that the above method, due to various effects, including aging and routine valve maintenance, can be highly ineffective, often resulting in stem thrusts at actuator shut off which are well below the required minimum thrust or well in excess of the maximum thrust that the valve is capable of withstanding. Excess thrust can cause the valve stem to bend, the valve seats to be damaged, the valve disc or body to be distorted, and even render the valve inoperable, often to the point where the valve is stuck closed and manually inoperable.
A malfunctioning valve at the very minimum can cause increased operating and servicing expenses, and in the worst possible scenarios create life-threatening conditions. One example of the former would be in a manufacturing plant where a water supply valve does not fully close because insufficient thrust is supplied by the valve actuator to the valve stem, thus leading to leakage and waste. An example of the latter situation would be in a steam driven power plant where an emergency situation requires the shut down of a high pressure steam line, the failure to do so resulting in pipe rupture or the like.
It is therefore easy to see why there is great interest in developing a method and apparatus by which the proper operation of a valve can be simply and inexpensively determined.
The typical valve for which the present invention is especially suited includes a valve stem which applies thrust to a valve disc or body to seat the disc into a valve seat. The valve stem is threaded, to accept a valve stem nut, or drive sleeve, which rotates about the valve stem. The valve stem, valve disc and valve seat are housed in a valve housing, with one end of the valve stem extending externally from the housing. The drive sleeve is disposed about the external portion of the valve stem and supported at a fixed distance from the valve housing by a valve actuator. As the drive sleeve is rotated, the valve stem is caused to move upward or downward, depending upon the direction of rotation of the drive sleeve.
The valve actuator may include a motive source which is coupled to the drive sleeve through a transmission assembly. This transmission assembly often takes the form of a worm and worm gear. The worm gear is mounted for rotation with the drive sleeve. The worm is positioned with axis of rotation perpendicular to the axis of rotation of the worm gear. As the motive source rotates the worm, the rotation of the worm is transferred to the worm gear for rotation of the worm gear in a plane perpendicular to the plane of rotation of the worm. The rotation of the worm gear is then transferred to the drive sleeve which, in turn, rotates to raise or lower the position of the valve stem.
As the valve disc comes into contact with the valve seat, the valve stem presents an increasing amount of resistance to the rotation of the drive sleeve. This resistance is transmitted to the worm via the worm gear, and results in movement of the worm in a direction away from the motive source.
In order to further rotate the worm, the worm gear and the drive sleeve, the motive source must supply additional force to the transmission assembly.
When a motor is employed as the motive source, it is typically regulated by a torque limiter mechanism which, as discussed above, senses the operating condition of the actuator and disconnects power to the motor whenever a predetermined condition is attained. Typically, the position of the worm and spring pack assembly is the condition which is sensed. After the worm and spring pack assembly has moved a predetermined distance, as a result of the increased resistance presented by the valve stem, power to the motor is automatically disconnected.
The movement of the worm is biased by a spring, typically a Belleville spring, the amount of compression of which is proportional to the thrust being supplied to the valve stem. The compression of the Belleville spring is tracked by a mechanism such as a gear which in turn actuates a switch. The switch is settable so that its contacts are disengaged when it is displaced over a selected distance. The curve obtained by plotting supplied thrust versus switch position of a properly operating unit is then used to set the switches of other similar units. In theory, once a calibration curve is obtained for a properly functioning unit, other similar units can be set by simply adjusting the torque switch control to a position determined from the calibration curve. It would, therefore, follow that a torque limiter could be set out in the field by simply setting the correct position on the torque switch.
As a verification of the proper functioning of the torque limiter switch, a reading may be taken of the operating current of the motor at the point where the motor is deactivated by the torque switch. So long as such operating current falls within the range of 1.5 to 3.0 times the running current, the valve is assumed to be operating correctly. The running current of the motor is defined as the current supplied to the motor when the motor is operating under normal load for the particular valve, i.e., not in the mode where the valve stem is beginning to provide turning resistance to the worm gear.
Once a valve has been installed within a system, the predominant method of verifying that the valve is operating correctly is to set the torque switch position as specified by the manufacturer and to monitor the operating current of the valve in the above manner. In practice, this method has been found to be, at times, highly inaccurate and often leads to valve damage, as well as to improperly operating valves. This is true because the indirect measurement method used in this approach is often not responsive to such occurrences as the aging of components of the valve, such as the Belleville springs, reconfiguration of the actuator to a high speed or a low speed mode, servicing of the valve itself, such as repacking of the valve seal, and various other effects.
U.S. Pat. No. 4,570,903 to Crass details the above and other problems associated with such an approach for setting torque switches. In an attempt to overcome these problems, Crass discloses the coupling of a force measuring device, such as a load cell, to the externally accessible end of a valve stem. An actuating force is then applied to the valve stem by a valve actuator. The valve stem actuation is terminated when a predetermined state of the actuating means is reached, with the output of the load cell being observed at the point where operation of the actuating means was terminated. This information is used in adjusting the setting of torque limiter switch controls so as to terminate the operation of the actuator when the desired amount of thrust is supplied to the valve stem.
Although this approach does not require the disassembly of the valve stem, it suffers from a number of drawbacks. For example, the method of the Crass patent provides an indication of the thrust on a valve stem under static conditions, for example when the valve disc or body is seated in a valve seat. However, this would not provide an accurate measurement of thrust under dynamic operating conditions when the valve stem is being moved toward and away from the closed position in which the valve body is seated. The Crass method purports to determine the net force composed of all of the forces applied to the valve stem, including the thrust supplied by the motive supply and transmission as well as sliding resistance provided by valve packing. However the dynamic resistance from valve packing, bent valve stems, and the like is not understood to be determined by the Crass approach. Therefore, information concerning these dynamic performance affecting characteristics of an operating valve are not available from the Crass approach.
Secondly, in the approach described in the Crass patent, the valve stem is drilled and tapped to receive coupling elements to which the load cell is connected to mount the load cell in position on the valve stem. Drilling and tapping of the valve stem weakens the stem. In addition, for valves that are already in place and which lack the tapped opening for receiving these coupling elements, as a practical matter one would have to remove the valve stem in order to drill and tap the stem to receive the load cell coupling components. In this case, the time-consuming and expensive disassembly of the valve would be required to permit these modifications of the valve stem. Also, assume the valve is being operated in an environment in which contaminants are passing through the valve. In such a case the removal of the valve stem is further complicated by the fact that the valve body is contaminated and must be carefully handled to prevent harm to the environment and to workers who have to modify the valve stem. Consequently, the Crass approach cannot easily be used in retrofitting existing valves with the components used to implement the illustrated Crass approach.
As another prior art approach, strain gauges have been mounted to valve stems and/or yokes of valves with the outputs of the strain gauges being read. The strain gauge outputs ar then used to calculate the thrust on the valve stem. This approach provides some indication of both dynamic and static valve performance characteristics. However, it has proven inaccurate. That is, to calculate the thrust on the valve stem, the Young's modulus for the stem material is required in the calculations. The Young's modulus varies with the exact material that is used for the stem and also with the characteristics of the batch of metal used in any specific valve. Consequently, computations of thrust are simply not dependable enough to provide an accurate indication of the actual thrust being applied to a valve stem.
Therefore, a need exists for a method of accurately determining the thrust on a valve stem during both static and dynamic conditions without requiring disassembly of a valve nor the use of error-prone mathematical computations to calculate the theoretical thrust on a valve stem.